The present disclosure relates to a thin film transistor (TFT) using an oxide semiconductor and a display device including the thin film transistor.
Oxide semiconductors such as zinc oxide and indium gallium zinc oxide (IGZO) have superior characteristics as an active layer of a semiconductor device, and have recently been developed so as to be applied to TFTs, light-emitting devices, transparent electroconductive films, and the like.
For example, oxide semiconductor TFTs have higher electron mobilities than a TFT having an amorphous silicon (a-Si:H) channel that has been used in liquid crystal display devices, and are thus superior in electrical properties. In addition, even a channel formed at a low temperature around room temperature can be expected to have a high mobility.
However, oxide semiconductors do not have sufficient heat resistance. It is known that oxygen atoms, zinc atoms or the like are diffused in the oxide semiconductor to form a lattice defect by heat treatment in a TFT manufacturing process. This lattice defect creates a shallow impurity level to reduce the resistance of the oxide semiconductor layer. In a TFT having an oxide semiconductor active layer, accordingly, a drain current flows even though a gate voltage is not applied (normally-on operation or depletion operation). Thus, the threshold voltage is reduced as the defect level increases, and the leakage current is increased accordingly.
It is reported that hydrogen is an element that can create a shallow impurity level in an oxide semiconductor (for example, Cetin Kilic et al., “n-Type doping of oxides by hydrogen,” Applied Physics Letters, Vol. 81, No. 1, pp. 73-75, Jul. 1, 2002). Accordingly, hydrogen and other elements introduced in TFT manufacturing processes, as well as the lattice defect, are likely to affect the characteristics of oxide semiconductor TFTs. In transistors having oxide semiconductor channels, accordingly, the carrier concentration in the channel tends to increase, and the threshold voltage becomes negative easily.
Since it is difficult to form a P-channel in a TFT having an oxide semiconductor channel, the circuit is constituted of only N-channel transistors. In this instance, if the threshold voltage is negative, the circuit configuration becomes complicated undesirably. In order to solve this problem, it is suggested to control the threshold voltage. The threshold voltage is represented by the following equation:
      V    Th    =            ϕ      MS        -                  Q        f                    C        OX              +          2      ⁢                          ⁢              ϕ        f              +                            2          ⁢                                          ⁢                      ɛ            s                    ⁢                      ɛ            0                    ⁢                      qN            A                    ⁢          2          ⁢                                          ⁢                      ϕ            f                                      C        OX            
In the equation, VTh represents the threshold voltage, φMS represents the difference in work function between the gate electrode and the oxide semiconductor layer, Qf represents the fixed charge, COX represents the capacitance of the gate insulating layer, φf represents the Fermi level of the oxide semiconductor layer acting as a channel, NA represents the accepter density, ∈S represents the dielectric constant of the oxide semiconductor layer, and ∈0 represents the dielectric constant of vacuum.
The threshold voltage of a TFT may be varied by doping a portion of the channel at the interface between the channel and the gate insulating layer, or by changing the Fermi level of the oxide semiconductor acting as the channel by varying the proportion of the constituents of the oxide semiconductor (for example, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2007-519256 and Japanese Unexamined Patent Application Publication No. 2008-85048.